Numerous hydrogel compositions and their biomedical applications are known in the art in the form of films formed from those compositions. U.S. Pat. No. 5,567,435 to Hubbell et al. (1996) disclosed a biodegradable hydrogel used as tissue contacting materials and controlled-release carriers. U.S. Pat. No. 7,091,299, to Salamone et al. (2006) disclosed an optically transparent hydrogel as used in ophthalmic devices such as intraocular lenses, contact lenses and corneal inlays. The polymer compositions were produced through the polymerization of one or more siloxysilane monomers or the copolymerization of one or more siloxysilane monomers with one or more aromatic or non-aromatic non-siloxy monomers, hydrophobic monomers or hydrophilic monomers.
U.S. Pat. No. 7,071,274, to Fujisawa et al. (2006) disclosed a silicon hydrogel having high oxygen permeability and transparency and being suitable to an ophthalmic lens, ocular lens and contact lens. U.S. Pat. No. 7,091,283, to Muller et al. (2006) disclosed a hydrophilic hydrogel used for biomedical moldings, for example ophthalmic moldings such as contact lenses. The hydrophilic hydrogels are made from crosslinkable copolymers, which are obtainable by (a) copolymerizing at least one hydrophilic monomer having one ethylenically unsaturated double bond and at least one crosslinker comprising two or more ethylenically unsaturated double bonds in the presence of a chain transfer agent having a functional group; and (b) reacting one or more functional groups of the resulting copolymer with an organic compound having an ethylenically unsaturated group. Recently, U.S. Pat. Nos. 7,279,507 and 7,247,270, to Hu et al. (2007), U.S. Pat. No. 7,249,849 to Marmo et al. (2007), U.S. Pat. No. 7,201,481 to Rosenzweig et al. (2007), U.S. Pat. No. 7,084,188 to Lai et al. (2006), and U.S. Pat. No. 7,147,325 to Gotou et al. (2006) described applications of hydrogel in soft contact lenses, technical problems remaining in wearing contact lenses, and solutions for them using hydrogel technology.
U.S. Pat. No. 7,091,049, to Boga et al. (2006) described a biosensor having a metalized film upon which was printed (contact printed) a specific predetermined pattern of an analyte-specific receptor. U.S. Pat. No. 7,105,588 to Yang et al. (2006), described a screen printable hydrogel for medical applications. The screen printable hydrogel composition comprises (a) soluble or partially soluble polymer wherein the polymer is a copolymer, interpolymer or mixture thereof; (b) initiation system; (c) thickener; (d) water; and (e) solvent; with the proviso that the composition has a viscosity of greater than about 10 Pa·s. U.S. Pat. No. 7,045,366 to Huang et al. (2006), disclosed a photo-crosslinked hydrogel blend surface coatings, where the hydrogel provides an improved approach using blend to achieve high quality, uniform coatings with better commercial viability than other approaches including copolymerization. Dextran and acrylamide systems are preferred. Benzophenone groups can be used as photocrosslinking groups. Applications of such hydrogel coating include mass spectral analysis of biomolecular analytes such as proteins.
One of the most widely applied polymer to form hydrogels, especially in medical applications, is poly(hydroxyethyl methacrylate), i.e., PHEMA. It is well known that PHEMA is a biocompatible polymer and is biologically inert, and can be easily prepared through free radical polymerization with or without solvent by photo and thermal initiations. In most practical applications, PHEMA is synthesized in the presence of a small quantity of crosslinker to form a crosslinked PHEMA. The degree of crosslinking can be adjusted according to the application requirements of hardness and mechanical strength.
PHEMA swells in water to form a hydrogel. Various parameters such as temperature, pH, and concentrations of ionic species in the solution determine the swelling behavior of PHEMA. This behavior has been explored for biosensor applications. In particular, the crosslinked PHEMA is transparent in its hydrogel form, which has led to the commercial success of PHEMA-based soft contact lenses.
Due to its hydrophilicity, PHEMA hydrogel offers more comfortable wearing than its competitor materials such as silicon-based hydrogel contact lenses. PHEMA has also been found to be suitable materials for wound dressing, biosensors, artificial muscles, and artificial organs. U.S. Pat. No. 5,498,407, to Atlas (1996) described PHEMA fibers used in cosmetic compositions containing same. The cosmetic composition consists of PHEMA fibers or copolymer of PHEMA fibers wherein the monomer is selected from the group consisting of acrylic acid, methacrylic acid, crotonic acid, hydroxyethylmethacrylate, hydroxyethylacrylate, hydroxypropylacrylate, hydroxypropyl methacrylate, hydroxybutyl methacrylate, N-vinyl-2-pyrolidone, and neopentyl glycol dimethacrylate in a weight ratio of 0.5 to 15%.
In considering the feasibility of a biomaterial for biomedical applications, for example, PHEMA based hydrogels used as contact lenses, the important parameters to be taken into account include water content and mechanical properties, which are closely related to the wearing duration and comfort of soft contact lenses. Unfortunately, attempts to improve both mechanical properties and water swelling content have been problematic. The water content of a PHEMA based polymers may be increased by introducing monomers of higher hydrophilicity into the PHEMA backbone, but this leads to poor mechanical strength of biomaterial making the resulting products less durable. On the other hand, the mechanical properties along with thermal stability can be improved by adding a higher concentration of crosslinker. However, the formed biomaterial with high ratio of crosslinker to PHEMA will make the biomaterial more rigid and decrease water content significantly. Finding a suitable balance between the mechanical strength and the water content of PHEMA hydrogels is therefore very challenging, especially in developing PHEMA-based hydrogels for biological applications, for instance, as soft contact lenses.
It is therefore apparent from the above that there is a need for the development of a transparent hydrogel with improved light transmittance, high water content and good mechanical strength.